1. Field of the Invention
The present invention relates to a method for the gravimetric mass metering of bulk solids and a differential metering scale suitable therefor.
2. Description of the Background Art
As is generally known, differential metering scales, also known as loss-in-weight scales, are often used for the gravimetric mass metering of bulk solids. The principle thereof, cf., e.g., Vetter: “Handbuch Dosieren” [Metering Handbook], ISBN 3-8027-2199-3; Chapter “Differential metering scales,” is physically very simple and theoretically free of error.
There are two major problems, however, with traditionally designed differential metering scales.
On the one hand, the container must be refilled after some time, which as a rule occurs at an unknown feed rate. During this time, the feed rate from the container cannot be determined from the change in weight over time and the changes in bulk density in the discharge device result in errors in the mass flow.
On the other hand, the container weight can be determined only with measurement errors. These errors must therefore be suppressed, so that they do not lead to an incorrect control signal in the metering unit. To this end, a low-pass filter is usually employed as a differential filter. However, the greater the interferences in relation to the weight loss due to the bulk solids discharge, the lower the cut-off frequency of the filter has to be set. This setting leads to a considerable reduction in the correction speed for rapid changes in the bulk density in the discharge or conveying device, which discharges or conveys away the bulk solids coming out of the container. This problem occurs even more pronounced in the metering of adhering and fine-grained bulk solids with a low feed rate and in addition relatively large differential metering scales with a high tare load.
The mentioned problems arise particularly in the case of differential metering scales in which the gravimetric discharge or feed output has a relationship, varying greatly over time, to the volumetric discharge output because of the flow properties. This is particularly the case in highly adhering or fine-grained bulk solids, which can be discharged with variable success from the container. These problems are known to users of differential metering scales and a solution to the problems has been long sought. Because of the noted problems with differential metering scales, there are very many other measuring techniques on the market for the flow measuring of bulk solids, e.g., belt scales, screw scales, Coriolis measuring systems, measuring systems utilizing changes in momentum in the bulk material flow, and measuring techniques that utilize other physical effects, to determine weight, e.g., electromagnetic techniques, cf., e.g., also Vetter: “Handbuch Dosieren,” ISBN 3-8027-2199-3; Chapter “Flow Metering Devices for Bulk Solids”. The publications DE 44 06 046 C2, WO 2008/055485 A1, U.S. Pat. No. 3,635,082, DE 20 18 618 A1, DE 33 15 476 A1, EP 0 669 522 A2, and DE 10 2006 052 637 A1 also describe different devices and methods for measuring the flow of media.
All of these measuring systems are either very costly to produce, when they are to have only low measurement errors or have a cross-sensitivity for other parameters as a matter of principle because of the employed physical effect; this then leads to considerable measurement errors. Efforts have already been made in many techniques to reduce the measurement errors to the necessary degree at considerable cost and with some limited success.